The process for manufacturing paper conventionally comprises three principal steps: (1) forming an aqueous suspension of cellulosic fibers, commonly known as pulp; (2) adding strengthening and/or sizing materials; and (3) sheeting and drying the fibers to form the desired cellulosic web.
Wood, the most widely utilized source of cellulose pulp, contains a mixture of compounds known as extractives, which are composed of a complicated mixture of various rosin acids, fatty acids, fats, waxes, and other low molecular weight neutral compounds. The specific composition of the extractives varies with the wood species.
During the manufacture of unbleached wood pulp by any of the well-known alkaline processes, the free acids and esters found in the wood extractives are converted to surface active, sodium salts of fatty and resin acids. These materials are commonly referred to as tall oil soaps. In the acid and mechanical pulping processes, the extractives remain essentially unchanged, but some of these compounds may be carried into the papermaking process as a result of incomplete washing.
Fatty acids and other surface active, carboxyl compounds can also be introduced into pulp as a result of the addition of defoamers, wetting agents, retention aids and wire cleaners. Such addition at several locations can result in high levels in the pulp. Fatty acids and other surface active, carboxyl compounds can also be introduced into pulp during the de-inking process practiced during the recycling of some printed papers. When these surface active, carboxyl compounds are present in the paper making process, they will be present in the liquid phase and adsorbed onto the fiber surface, as free acids, sodium salts, or as salts of divalent metal ions.
Tall oil soaps and other surface active materials are known to adversely affect the strength of paper and the performance of strength additives when present in the papermaking system, even when present at levels as low as 0.05% (Worster, H. E., et.al. TAPPI 63(11) 63 (1980), Bruun, H. H. Svensk Papperstidning 78(14)512(1975), Springer, A. M., et. al. TAPPI Journal 69(4)106(1986), Brandel, J., and Lindheim, A., Pulp and Paper Mag. Can. T-431 (1966)). Normally, the pulps containing surface active, carboxyl compounds at levels sufficient to interfere with the performance of strength-enhancing additives, such as acrylamide copolymers, are unbleached pulps.
An improved process for making paper with increased strength using unbleached pulp containing soluble anionic materials, also known as anionic trash, using a combination of a water-soluble linear cationic polymer having a high molecular weight and a water-soluble anionic polymer that is reactable in the presence of water with the cationic polymer to form a polyelectrolyte complex, is disclosed in U.S. Pat. No. 5,338,406, which is incorporated herein by reference in its entirety. However, when used with some pulps, particularly those containing tall oil soaps and other surface active materials, that process is not fully effective to make paper with sufficient strength.
Tall oil soaps and other surface active carboxyl compounds are well known to interact with multivalent cations to form metal soaps; see, for instance, Allen, L. H., TAPPI Journal 71(1) 61 (1988), and Young, S. L., and Matijevic, E. J. Colloid Interface Sci. 61(2) 287 (1977)). The products of these interactions, particularly those involving aluminum ions derived from alum, have found many uses in the paper industry.
On the other hand, addition of alum, or alum in the presence of surface active carboxyl compounds, to a papermaking process is well known to have an adverse effect on the strength properties of the paper. This is particularly true when high levels of these materials are added. (See Worster, H. E., et.al. TAPPI 63(11) 63 (1980)). It is on this basis that papermakers generally try to minimize the amount of alum they use.
Alum is aluminum sulfate, Al.sub.2 (SO.sub.4).sub.3, with various amounts of water of hydration. It is widely employed in the paper industry to fix rosin size, increase drainage, improve retention, and reduce anionic charge. For example, alum is widely used in combination with rosin, a component of tall oil, to make a size for paper. The rosin aluminate formed by the interaction between these two materials adsorbs on the fiber surface and renders it hydrophobic. In unbleached papermaking systems, it is normally employed for these purposes at addition levels less than 1%. An excellent review of this chemistry can be found in Davison, R. W. TAPPI 47(10) 609 (1964). Alum has sometimes also been recommended as a pitch control agent (See Back, E., Svensk Papperstidning 59(9) 319 (1956), and Allen, L. H., TAPPI 63(2)81(1980)).
Alum is also used in combination with anionic acrylamide copolymers, to improve the dry strength of paper (Azorlosa, Canadian Patent No. 477,265) where it acts as a retention aid for these anionic copolymers. Alum may also be used in papermaking systems where cationic resins are used, for example, as a component of the sizing system, as a dye fixative, or as a drainage aid (Reynolds, W. F. in "Dry Strength Additives", Tappi Press. Atlanta, Ga. 1980. Chapter 60). For instance, alum has been employed along with the certain cationic, hydrophobic dry strength additives disclosed by Strazdins in U.S. Pat. No. 3,840,489 to neutralize the soluble anionic material found in unbleached pulp. That material has been shown to interfere with the ability of the resin to enhance strength (Strazdins, E. International Seminar of Paper Mill Chemistry, Amsterdam 1:26p, Sep. 11-13, 1977).
It has now been discovered that the use of alum in conjunction with certain cationic and anionic polymers can improve the strength of paper made from pulps containing surface active carboxyl compounds.